1. Field
One embodiment of the present invention relates to an optical disc (or an information storage medium using light in general concept) by which information can be recorded on or reproduced from two or more recording layers of the one side of the disc, a recording method using the disc, and a reproducing method using the disc.
2. Description of the Related Art
As an optical disc, in general, there are a read-only ROM disc, a recordable or re-recordable R disc, and a rewritable RW or RAM disc. As information becomes bulky, further-large capacity is demanded for an optical disc. For the purpose of increasing the capacity of an optical disc, some technique has been proposed in which a recording capacity is increased by narrowing down a beam spot, for example, in such a manner that a wavelength of a laser beam is shortened, or a numerical aperture NA is enlarged (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 2004-206849, paragraphs 0036 to 0041, FIG. 1).
As multi-layered optical discs, dual-layer ROM discs are conventionally available in the market. Recently, dual-layer recordable discs (DVD-R:DL) each using a laser of 650 nm wavelength are reduced to practice. In a manner of recording and reproducing an optical disc (such as a DVD-R) using an organic dye material for the recording layer, recording marks in which the reflectivity of the dye has been changed are formed by modulating the power of a laser light. Thus, the information recording is performed utilizing the difference between the reflectivity of recording marks and that of unrecorded portions. As a manner of modulating the laser power, multi-pulses are used for DVD-R, for example (cf. Jpn. Pat. Appln. KOKAI Publication No. 9-282660, abstract).
A single-layer recordable R disc configured to perform recording with a laser wavelength of 405 nm is prepared, and investigation is made for information recording on the disc. It is found that the bias power, which is not so significant matter with the recording of 650 nm laser wavelength, is very significant matter to control the recorded mark length. Thus, the bias power has an effect on recording characteristics. From this, the number of parameters of the recording condition increases, to thereby consume much time to find an optimum recording condition. The inventors discover that an optimum recording condition (for instance, a condition serving to determine the recording power to achieve a minimum error rate where the wavelength and/or the waveform is/are fixed) can be found in short time while changing the recording power with a constant ratio of the recording power (or the peak power of recording pulses) to the bias power, as a given condition.
However, when similar information recording is performed for a dual-layer R disc, a problem occurs. That is, the inventors are faced with a problem that when the ratio of the recording power to the bias power is fixed and recording is performed for each of the layers, even if both the layers are formed of the same recording-layer material, the recording characteristic of one of the layers becomes significantly deteriorated.
Another problem is also found. That is, when an information storage medium (especially a multi-layer optical disc configured to perform high-density recording with a blue laser) is preserved for a long period of time, and/or when the medium is preserved in severe circumstances such as high-temperature and high-humidity, a sufficient time is consumed to find the optimum recording condition, because the optimum recording power may change after the information storage medium is newly manufactured.